Electrolytic treatment of metal surfaces



-hexavalent chromium ions.

United States Patent This invention relates to the electrolytic treatment of metal surfaces in order to enhance the corrosion resistance and paint adherence thereof.

This application is a continuation-in-part of our application Serial No. 815,782, filed May 26, 1959, now abandoned.

In known metal treatment processes of the prior art, the use of chromic acid is well known to industry. According to- US. Patent Nos. 2,768,l034, the formation of a thin film consisting of chromium chromate on steel sheets is effected by a process which comprises subjecting a part of the chroinate on the surface thereof to a reducing action by means of a reducing agent. US. Patent No. 2,780,592 teaches that a ferrous metal article can be cathodically electrolyzed in a solution containing 100-400 g./l. of chromic acid (50400 g./l. hexavalent chromium ion) having added thereto more than 8 g./l. of boric acid ions at room temperature, but a period of seconds or more is required to complete the treatment.

In addition, a phosphating process is also well known to industry in which an acid solution of iron, zinc, manganese, and calcium phosphates is employed. The familiar processes of today, however, are slow and have proved troublesome to control and operate, and sometimes produce the formation of sludges in the solution thereof.

Th'is'process also has the disadvantage that the corrosion resistance thus obtained has proved unsatisfactory.

It is also known that to cathodically electrolyze a ferrous metal in a solution containing hexavalent chromium ions and phosphate ions and having a pH value of from 1 to 2, which, however, results in the formation of chalky films providing inadequate corrosion resistance depending upon the treating conditions, particularly at lower bath temperatures, and that therefore this process has the disadvantage that it cannot be carried out at room temperature. I

Phosphating processes of prior art consist in producing a protective film by the use of a corrosion reaction of iron. In this reaction the presence of hexavalent chromium ions tends to passivate iron so that corrosion is inhibited and the ferrous compound dissolved in the solution is oxidized-to the ferric compound, which exerts an adverse effect on the precipitation of excellent phosphate coatings. Accordingly, the addition of chromic acid to the phosphating solution has not been widely adopted.

The process of the invention contemplates precipitating phosphates from the solution by electrolysis, another than employing the corrosion reaction of iron mentioned above, and hence the above disadvantages are obviated, although a solution containing an appreciable amount of hexavalent chromium ions is used.

The cathodic electrolytic treatment of the prior art in the hexavalent chromium ionphosphate ion solution tends to produce chalky films at a low temperature, which, it is believed, results from the precipitation of a chalky film consisting of chromium phosphate which does not adhere to the metal surface. The chromium phosphate thus obtained is believed to result from the reaction of phosphate ions in the solution with trivalent chromium ions produced by the cathodic reduction of It follows that the above reaction may inhibit the formation of chromium chro- 3,118,824 Patented Jan. 21, 1964 ICC mate sol which exerts a beneficial effect on the enhancement of corrosion resistance. Accordingly, corrosion resistance may be lowered.

On the contrary, however, in the process of this invention, chalky films are not formed when the treatment is carried out at room temperature. Therefore an effective electrolytic treatment can be carried out even at a low temperature. It is considered that when a sufiieient amount of zinc, or magnesium, or calcium or manganese ions is included in the solution of the invention, the application of phosphate coatings to metal surfaces is effected by the action of these cations, and a large part of the trivalent chromium ions which have been reduced may serve for the formation of chromium chromate sol.

In accordance with the invention, a metal article is cathodically electrolyzed in an acid solution containing zinc, or magnesium, or calcium, or manganese ions, the concentration of phosphate ions being much higher than that of the prior art, for a very short period of time in order to impart a corrosion resistance as well as a lacquer adherent film to the metal article. In this treatment process, no sludge forms in the solution, and the control of the solution is easy. Furthermore, since no chalky film is produced at a low temperature, this process can be performed at room temperature.

Compared with an iron or other metal article treated by the conventional phosphating process, the same article treated by the process of the invention exhibits corrosion resistance so excellent that it would not be corroded by salt spray after a period of more than six hours in a salt water spray test (the concentration of salt in water, 5%; the temperature within the test tank, 35 C.; and spray pressure, 20 p.s.i.), and has lacquer adhesion as strong as articles treated by prior phosphating methods, including strong resistance to corrosion of the base metal under the lacquer or paint coatings.

An embodiment of the invention is described in detail hereinbelow.

. It is necessary thatthe treating solution to be used should contain 50 to g./l. of phosphate ions, 5 to 25 g./l. of hexavalent chromium ions and the cations of a material selected from a group consisting of zinc, magnesium, calcium and manganese. In preparing the solution, the phosphate ions can be supplied by phosphoric acid and the phosphate of any of the above mentioned cations, and the hexavalent chromium ions can be supplied by chromic anhydride and the chromate or dichromate of any of the above mentioned cations.

The cations of zinc and others can be supplied by the salts of such weak acids as oxides and carbonates. ln preparing the solution, in case the salts of such cations are used for supplying hexavalent chromium ions and phosphate ions, their existing amounts must be taken into consideration.

The pH of the solution is important. In order to obtain afavorable result, the pH of the solution must be kept from 1 to 2.5 (most perferably from 1.5 to 2.5). If the pH is lower than that, the anticorrosiveness and lacquer adhesiveness will deteriorate. If it is higher than that, a sludge will be produced in the solution and thus the stability will deteriorate.

The amounts of these cations giving the above mentioned pH range are different depending on their kinds and can not be stoichiometrically determined. However, according to the results of our experiments, within the above mentioned range of hexavalent chromium ions and phosphate ions, the favorable range of the concentration of zinc ions is 8 to 40 g./l. as Zn++, that of magnesium ions is 6 to 30 g./l. as Mg that of calcium ions is 6 to 30 g./l. as Ca++, and that of manganese ions is 7 to 35 g./l. as Mn Steel sheet can be cathodically electrolyzed in the above aqueous electrolytic solution at a current density of from to 300 amps per sq. ft. at a temperature of from 10 to 80 C. in a period of 0.25 second or more. The treated steel sheet removed from the solution can be squeezed in order to remove any excess solution pressent thereon, and it is then dried, which results in the formation of a corrosion resistant as well as paint adherent film thereon. As to drying, it may be exposed to air for drying, but it is more preferable that it be subjected to forced air drying at a temperature of from 100 to 200 C. for a period of from 2 to 5 minutes.

The exact nature of the reaction or the formation of the coating film is not known precisely, but it is believed that the cathodic electrolytic action to which the metal is subjected produces hydrogen on the surface of the metal and at the same time the reducing reaction of the hexavalent chromium ions takes place. Accordingly, the pH value on the metal surface increases temporarily, and the phosphate of the cations (i.e., Zn or Mg or Ca or Mn) precipitates on the metal surface from the solution to form a closely adherent thin film which effectively holds the sol-like deposit consisting of the mixture of the chromium chromate produced by the partial reduction of hexavalent chromium ions and the phosphate ions remaining in the solution, which results in a thin uniform film covering the entire metal surface. On drying, the sol-like film turns into a gel to form a water-repellent film so as to improve its corrosion resistance as well as its paint adhesion. In this case, it seems that the above phosphate ions serve not only for the formation of the phosphate coating film, but also for the strengthening of this film due to its remaining in the gel of the chromium chromate.

Accordingly, zinc, magnesium, calcium, or manganese ions, phosphate ions, hexavalent chromium ions, and

cathodic electrolysis are absolutely required in the process of the invention. The absence of any one of the above three important factors has resulted in an inferior protective coating as compared to that produced by the method of the instant invention.

When the electrolytic solution containing zinc ions is employed in the process of the invention, it would seem that the electrodeposition of zinc would be effected by the cathodic electrolysis, but, as a matter of fact, zinc does not deposit because of the low concentration of zinc and also because of the acid solution, which promotes essentially the evolution of hydrogen and the cathodic reduction of hexavalent chromium ions.

Our invention will be more clearly understood by referring to the following example.

Example 1 Zinc-plated metal is cathodically electrolyzed according to the following.

Solution: G./l.

Chromic anhydride (containing 7.5 g./l. hexavalent chromium ions).

Phosphoric acid 70 (containing 68 g./l.

phosphate ions).

Zinc oxide 23 (containing 18.5

g./l. zinc ions).

Temperature of bath Room temperature. Current density 80 amps per sq. ft. Treating time 1 second.

Example 2 Steel is cathodically eelctrolyzed according to the following.

Solution: G./l.

Chromic anhydride 10 (containing 5 g./l. hexavalent chromium ions).

Phosphoric acid 50 (containing 48.5 g./l. phosphate ions).

Zinc oxide 18 (containing 14.4

g./l. zinc ions).

Temperature of bath Room temperautre. Current density 60 amps per sq. ft. Treating time lsecond.

Example 3 Steel is cathodically electrolyzed according to the following.

Solution: 0/].

Chromic anhyrdide 15 (containing 7.5 g./l. hexavalent chromium ions).

Phoshporic acid (containing 68 g./l.

phosphate ions).

Magnesium oxide 25 (containing 15 g./l.

' magnesium ions). Temperature of bath Room temperature. Current density 60 amps per sq. ft. Treating time lsecond.

Example 4 Steel sheet is cathodically electrolyzed according to the following.

Solution: G./l.

Chromic anhydride 15 (containing 7.5 g./l. hexavalent chromium ions).

Calcium orthophosphate,

Ca(H PO -H O (containing 60 g./l. phosphate ions and 12.7 g./l. calcium ions). Temperature of bath 50 C. Current density 60 amps per sq. ft. Treating time 1 second.

Example 5 Steel is cathodically electrolyzed according to the following. Solution: G./l.

Chromic anhydride 20 (containing 10 g./l. hexavalent chromium ions).

Phosphoric acid 60 (containing 58 g./l.

phosphate ions).

Manganese carbonate 35 (containing 16.7 g./l.

manganese ions).

Temperature of bath 50 C. Current density 60 amps per sq. ft. Treating time 1 second.

As fully described in the foregoing, the process of the invention can be used not only for steel sheet, but also for other ferrous metals and non-ferrous metals as well, since this process contemplates precipitating the film consisting of phosphate and chromium chromate on the metal surface by means of the electrolytic treatment, not by the dissolving reaction of the metal.

We claim:

1. A surface treating method for metallic products comprising the steps of placing the metallic product in an electrolyte which is an aqueous solution containing from 5 to 25 g./l. hexavalent chromium ions, 50 to g./l. phosphate ions, and cations from a material selected from the group consisting of zinc ions as Zn++ in an amount of from 8 to 40 g./l., magnesium ions as Mg in an amount of from 6 to 30 g./l., calcium ions as Ca1+ in an amount of from 6 to 30 g./l. and manganese ions as Mn in an amount of 7 to 35 g./l., in an amount such that the pH of the solution is from 1 to 2.5, and while keeping the temperature of the solution from 10 to 80" C.. passing a current through the solution with the metallic product as the cathode and at a current density of from 10 to 300 amps/sq. ft. for a period of longer than 0.25 sec.

2. A surface treating method for metallic products comprising the steps of placing the metallic product in an electrolyte which is an aqueous solution containing from 5 to 25 g./l. hexavalent chromium ions, 50 to 100 g./l. phosphate ions, and from 8 to 40 g./l. zinc ions as Zn++ in an amount such that the pH of the solution is from 1 to 2.5, and while keeping the temperature of the solution from 10 to 80 C., passing a current through the solution with the metallic product as the cathode at a current density of from 10 to 300 amps/sq. it. for a period of longer than 0.25 sec.

3. A surface treating method for metallic products comprising the steps of placing the metallic product in an electrolyte which is an aqueous solution containing from 5 to g./l. hexavalent chromium ions, 50 to 100 g./l. phosphate ions, and from 6 to g./l. calcium ions as Ca++ in an amount such that the pH of the solution is from 1 to 2.5, and while keeping the temperature of the solution 4. A surface treating method for metallic products comprising the steps of placing the metallic product in an electrolyte which is an aqueous solution containing from 5 to 25 gl/l. hexavalent chromium ions, to 100 g./l. phosphate ions, and from 6 to 30 g./l. magnesium ions as Mg in an amount such that the pH of the solution is from 1 to 2.5, and while keeping the temperature of the solution from 10 to C., passing a current through the solution with the metallic product as the cathode at a current density of from 10 to 300 amps/sq. ft. for a period of longer than 0.25 sec.

5. A surface treating method for metallic products comprising the stcps of placing the metallic product in an electrolyte which is an aqueous solution containing from 5 to 25 g./l. hexavalent chromium ions, 50 to g./l. phosphate ions, and from 7 to 35 g,/l. manganese ions as Mn++ in an amount such that the pH of the solution is from 1 to 2.5, and while keeping the temperature of the solution from 10 to 80 C., passing a current through the solution with the metallic product as the cathode at a current density of from 10 to 300 amps/sq. ft. for a period of longer than 0.25 sec.

References Cited in the file of this patent UNITED STATES PATENTS 1,007,069 Coslett Oct. .31, 1911 2,746,915 Giesker et al May 22, 1956 2,812,296 Neish Nov. 5, 1957 

1. A SURFACE TREATING METHOD FOR METALLIC PRODUCTS COMPRISING THE STEPS OF PLACING THE METALLIC PRODUCT IN AN ELECTROLYTE WHICH IS AN AQUEOUS SOLUTION CONTAINING FROM 5 TO 25 G./L. HEXAVALENT CHROMIUM IONS, 50 TO 100 G./L. PHOSPHATE IONS, AND CATIONS FROM A MATERIAL SELECTED FROM THE GROUP CONSISTING OF ZINC IONS AS ZN++ IN AN AMOUNT OF FROM 8 TO 40G./L., MAGNESIUM IONS AS MG++ IN AN AMOUNT OF FROM 6 TO 30G./L., CALCIUM IONS AS CA++ IN AN AMOUNT OF FROM 6 TO 30G./L. AND MANGANESE IONS AS MN++ IN AN AMOUNT OF 7 TO 35G./L., IN AN AMOUNT SUCH THAT THE PH OF THE SOLUTION IS FROM 1 TO 2.5, AND WHILE KEEPING THE TEMPERATURE OF THE SOLUTION FROM 10 TO 80*C., PASSING A CURRENT THROUGH THE SOLUTION WITH THE METALLIC PRODUCT AS THE CATHODE AND AT CURRENT DENISTY OF FROM 10 TO 300 AMPS/SQ. FT. FOR A PERIOD OF LONGER THAN 0.25 SEC. 